1. Field of the Invention
This invention relates in general to a device for identifying an individual by verification of his or her fingerprint. The invention relates in particular to a fingerprint sensor for transforming the information presented by the papillary structure of a finger into an electrical output signal.
2. Description of the Prior Art
Fingerprint identification systems which identify the crest-and-valley configuration of a finger pressed on a contact surface are well known in the art.
In various systems, the finger press is interrogated by a light beam directed through the front surface of a transparent finger bed. The interrogating beam is partially reflected at the back surface in accordance with the optical discontinuities created by the finger pressed on this surface. Thus, the reflected light contains finger press information which can be received by an opto-electric element. Such a sensor which is disclosed for example in U.S. Pat. No. 4,053,228 requires beam-scanning and focusing devices and is, therefore, relatively voluminous and complicated. Beam directing means are avoided, if the assembly is varied in the following manner: Radiation is passed through a plate-shaped finger bed by internal total reflection. The plate appears homogeneously dark for an array of photosensitive elements which extend in a plane parallel to the finger bed plate. Once the finger is pressed on the plate, the resilient plate surface is distorted and causes a light pattern corresponding to the impressed relief. This assembly which is described in the co-pending U.S. patent application Ser. No. 176,696 filed on Aug. 11, 1980 and assigned to the same assignee (now U.S. Pat. No. 4,340,300 issued July 20, 1982) can be implemented as a small sized unit composed of only a few parts. Yet, it has still the drawbacks common to all sensors operating with light: the need for using a power-consuming radiation source and optical elements such as filters or lenses to create an evaluable image of the finger relief.
Therefore, efforts have been made to replace light by another transducing medium. For instance, in U.S. Pat. No. 3,622,989 there is shown a fingerprint detection system in which a finger connected to a common bar is pressed against an array of sense electrodes and conductive contact is made between the contact bar and selected sense electrodes via the ridges of the finger surface. Using the finger's surface conductivity for generating a current distribution is theoretically a very promising approach. However, in practice it is difficult to achieve reproducible results. Virtually any impurity such as moisture or dust on the finger under interrogation or on the contact surface of the electrode array can adulterate the current distribution.
In the co-pending U.S. patent application Ser. No. 170,606, filed on July 21, 1980 and assigned to the same assignee (now U.S. Pat. No. 4,394,773 issued July 19, 1983), there is discussed a sensor for changing the pressure pattern of a fingerprint into a charge pattern by means of a piezoelectric substance. The charge pattern is subsequently measured by a charge-coupled device (CCD) matrix, for example, of the type disclosed in the Preliminary Data Sheet for a Solid State Image Sensor Array RA 100.times.100; EG & G Reticon, Sunnyvale, Calif.